Can't build by default on macOS

[jdm]

error[E0433]: failed to resolve: could not find `universal` in `generic`
  --> src/lib.rs:45:33
   |
45 | use surfman::platform::generic::universal::context::Context;
   |                                 ^^^^^^^^^ could not find `universal` in `generic`

We need to either enable sm-osmesa by default in Cargo.toml or add a feature that enables it.

[asajeffrey]

Yes, at the moment I build with --features=surfman/sm-osemsa, but we could expose a feature for that. This is all probably temporary until the API for bootstrapping settles down.